Hydraulic cements are known which are composed of materials formed by oxides and defined chemicals from the field of inorganic chemistry, the use of which is based on the chemical reaction of the oxides and other constituents of the cement with water in order to form hydrated compounds which aggolmerate to form a mass wherein more or less rapid mechanical strengths are developed. Within the general concept of "hydraulic cements" there are on the market cements of the above-mentioned type, some capable of very fast development of mechanical strength. These are broadly known as "aluminous cements".
Said aluminous cements are substantially composed of calcium aluminate of the formula CaO.Al.sub.2 O.sub.3 and minor amounts of calcium aluminoferrite 4 CaO.Al.sub.2 O.sub.3. Fe.sub.2 O.sub.3, as well as traces of other calcium aluminates of the formula 12CaO. 7Al.sub.2 O.sub.3.
The invention of this application is not related to such aluminous cements but rather to those cements based on the calcium sulfoaluminate compound of the formula 4CaO.3Al.sub.2 O.sub.3.SO.sub.3.
The oldest reference to cements based on calcium sulfoaluminate was made in a French patent granted in 1936 to the firm Poliet et Chausson, Paris, France, which invention had its origin in the research of Henri Lossier. Those cements, based on calcium sulfoaluminate, have since then become known as Lossier cements.
By 1941 the above-mentioned French firm was manufacturing upon request "expansive and contractionless cements", produced from a clinker obtained through calcination of a mixture of suitable compositions, based on gypsum, clays and limestone.
By 1958, in the United States, Klein submitted a paper during the 61st Annual Meeting of the ASTM, wherein he stated that a compound, non-precisely identified (which seemed to correspond to an anhydrous calcium sulfoaluminate), was the cause of the behavior of these expansive cements. Since then practically, such expansive cements based on calcium sulfoaluminate are known as K cements by ASTM and follow specification No. C-845: Expansive hydraulic cements.
Expansive cements, such as those mentioned above, were manufactured in a more or less empiric way until the Fifth International Congress on the Cement Chemistry, held at Tokyo in 1968; where in the lectures of various authors there was agreement identifying the main component of such cements as being calcium sulfoaluminate (having the formula 4CaO.3Al.sub.2 O.sub.3.SO.sub.3.
According to the prior art teachings to obtain such expansive cements, the clinker based on calcium sulfoaluminate was admixed with Portland clinker, as well as with granulated slag from blast furnaces, in order to obtain a "controlledexpansion" product. Since then, the cement researchers became aware that those cements based on calcium sulfoaluminate clinker exhibited earlier and greater strengths relative to those obtained with the use of Portland cement alone (as was clearly established, for instance, in U.S. Pat. No. 4,957,556 to Hassan Kunbargi).
Regarding the Kunbargi invention and according to the teachings derived therefrom, it is practically impossible to determine accurately the setting time of the cement by the Vicat method, as the setting time is very fast, thus making the manipulation of the concrete produced from such cement very difficult, even though the use of citric acid is recommended as a setting retardant.
Another problem of the cement produced according to the Kunbargi patent is that the development of the strength thereof is unpredictable and, although in certain instances the anticipated compressive strengths can be attained, frequently much lesser strengths are obtained. This problem is due basically to those materials forming the Kunbargi cement, as defined in the corresponding U.S. Patent.
Also regarding the Kunbargi patent, it is pointed out at columns 3 and 4 that the calcium sulfoaluminate is not thermodynamically stable, and when heated in an industrial kiln, decomposes at temperatures higher than 1200.degree. C.
On the other hand, the Kunbargi patent clearly states at column 5 that, due to the thermodynamic properties of the calcium sulfoaluminate, it is highly difficult, if not impossible, to obtain clinker resulting from calcination of the raw materials to produce cement containing effective amounts of calcium sulfoaluminate, tricalcium silicate and dicalcium silicate. As a consequance of which, it is pointed out in said patent that clinker thus produced must be admixed with a separate hydraulic clinker containing tricalcium silicate and dicalcium silicate.
Finally, with respect to the Kunbargi patent, there are doubts about the fact that the compressive strengths mentioned therein are actual; particularly when it is assumed that through the use of the test methods identified as ASTM-C-109 or EN196-1 (the latter being the standard for the European Community), the required values for such strengths are not obtained. It is also not clear what would be the actual value for such strength, since in the context of the above-recited patent the strength of the cement produced according to the Kunbargi method is compared to the strength of Portland cement.
Furthermore, with respect to the problem of measuring the strength as developed by Kunbargi cement, it is not clear which is the method he used to determine the cement setting at different ages thereof; which leads one to believe that such measurements are equivalent to those made regarding Portland cements; and in any case he states that they are equivalent. This in fact is not true, since the value of the mechanical strength of any type of hydraulic cement is not an absolute value (such as might be said for the specific gravity or the chemical composition thereof). On the contrary, the values for such strength basically depend on the method employed to evaluate the same. Mechanical strength, in case of hydraulic cements, is a function of the water:cement ratio as well as of the cement:sand ratio. Thus, when any of said ratios are changed, the results in the measuring methods are not comparable.
According to the ASTM method, the cement:sand ratio is 1:2.75 and the water:cement ratio is about 0.49 for Portland cements. In Kunbargi's patent a cement:sand ratio of 1:1 is employed and, of course, the water:cement ratio that he used is not clear.